The present invention relates to the fields of measurement instruments, in particular to mass spectrometers used for analyses of substances based on results of determination of masses of their ions or spectra of masses.
In order to understand the present invention, terminology used in the description, and novelty of the present invention over the prior art, it would be advisable first to explain the general principles of construction and operation of mass spectrometers and their classification.
A mass spectrometer is an instrument for separation of ionized particles (such as atoms, molecules, cluster formations) by their masses, more specifically, by a ratio of ion mass m to its charge. The separation is carried out under the effect of magnetic and electric fields. Furthermore, mass spectrometer is used for determining masses of ions and relative contents of specific ions in a substance, i.e., spectrum of masses.
A typical mass spectrometer consists of the following parts: a system for preparation and introduction of an a substance to be analyzed into the instrument; an ions source where the aforementioned substance is ionized at least partially and where an ion beam is formed; a mass analyzer where the ions are separated in accordance with an m/e ratio, focused, and are emitted from the ion source in various directions within a small space angle; a ion receiver or collector where ion current is measured or converted into an electrical signal; and a device for amplification and registration of the output signal. In addition to amount of ions (ion current), the registration unit also receives information about ion mass. Other units included into a mass spectrometer are power supplies, measurement instruments, and a vacuum system. The latter is required for maintaining the interior of the mass spectrometer under high vacuum, e.g. of about 10xe2x88x923 to 10xe2x88x927 Pa. Operation is normally controlled by a computer, which also stores the acquired data.
A mass spectrometer is characterized by its resolution capacity, sensitivity, response, and a range of measured masses. The aforementioned response is a minimal time required for registration of mass spectrum without the loss of information within the limits of so-called decade of atomic mass units (1-10, 10-100, etc.). Normally such time is 0.1 to 0.5 sec. for static mass spectrometers and 10xe2x88x923 for dynamic (time-of-flight) mass spectrometers.
A substance to be analyzed is introduced into the mass spectrometer with the use of so-called molecular or viscous flow regulators, load ports, etc.
By methods of ionization, ion sources of mass spectrometers can be divided into various categories, which are the following: 1) ionization caused by collisions with electrons; 2) photo-ionization; 3) chemical ionization due to ionic-molecular reactions; 4) field ion emission ionization in a strong electric field; 5) ionization due to collisions with ions; 6)atomic-ionization emission due to collisions with fast atoms; 7) surface ionization; 8) spark discharge in vacuum; 9) desorption of ions under effect of laser radiation, electron beam, or products of decomposition of heavy nuclei; and 10) extraction from plasma.
In addition to ionization, in mass spectrometer an ion source is used also for forming and focusing an ion beam.
More detail general information about types and constructions of ion sources suitable for use in mass spectrometers can be found in xe2x80x9cIndustrial Plasma Engineeringxe2x80x9d by Reece Roth, Vol. 1, Institute of Physics Publishing, Bristol and Philadelphia, 1992, pp. 206-218.
By types of analyzers, mass spectrometers can be divided into static and dynamic. Static mass spectrometers are based on the use of electric and magnetic fields which remain, during the flight of ions through the chamber, practically unchanged. Depending on the value of the m/e ratio, the ions move along different trajectories.
The most popular static mass spectrometer is a conventional mass spectrograph in which ion beams with different e/m ratios are focused in different areas of a photo-sensitive element, e.g., a photo-sensitive plate located in a focal plane of the instrument. Since the outlet opening of the ion source is made in the form of a slit, after development, the points hit by ions are seen on such a plate in the form of strips. In a static mass spectrometer the ion beam is focused onto the slit of the ion receiver. If the electric or magnetic field varies smoothly, the ion beams with different e/m ratios will sequentially pass through the aforementioned slit. Continuous registration of the ion current will produce a graph with ion peaks on the mass spectrum. If necessary, the obtained mass spectrum can be used for quantitative evaluation by methods of photometry.
In a static mass spectrometer with a homogeneous magnetic field, ions are emitted from the ion-source slit in the form of a diverging beam. When the diverging beam enters the magnetic field, it is divided into beams with different elm ratios. Such beam can be registered on a photo-sensitive plate or in any other registering device. Static mass spectrometers, in turn, can be divided into various types such as static mass spectrometers with a non-homogeneous magnetic field, with ion prisms that separate the beam into sub-beams with different elm ratios, and with double-focusing of the ion beam. Various combinations of the aforementioned mass spectrometers are also possible.
It should be noted that static mass spectrometers are static installations which are heavy in weight, complicated in construction, an operation with them require the use of skilled personnel.
In dynamic mass spectrometers, the ions are separated on the basis of different times of flights through the given distance. Furthermore, the ions can be separated under the effect of pulse or RF electromagnetic fields with periods equal to or shorter than the time of flight of ions through the analyzer. Among the dynamic mass spectrometers, most popular ones are time-of-flight types, RF types, quadrupole types, magnetic-resonance type, and ion-cyclotron resonance types of mass spectrometers.
In time-of-flight mass spectrometers, ions formed in the ion source are injected into the analyzer via a grid in the form of short pulses of ion current. The analyzer comprises an equipotential space. On its way to the collector, the pulse is decomposed into several sub-pulses of the ion current. Each such sub-pulse consists of ions with the same elm ratios. The aforementioned decomposition occurs because in the initial pulse all ions have equal energies, while the speed of flight V and, hence, the time of flight t through the analyzer with the length equal to I are inversely proportional to m1/2:
T=I(m/2eV)1/2. 
A series of pulses with different e/m ratios forms a mass spectrum that can be registered, e.g., with the use of an oscilloscope. Resolution capacity of such an instrument is proportional to length l.
An alternative version of the time-of-flight mass spectrometer is a so-called mass-reflectron, which allows an increase in resolution capacity due to the use of an electrostatic mirror.
Energies of ions collected in each packet are spread over the temperature of the initial gas. This leads to broadening of peaks on the collector. Such broadening is compensated by the electrostatic mirror that prolongs the time of flight for slow ions and shortens the time of flight for fast ions. With the drift path being the same, the resolution capacity of a mass reflectron is several times the resolution capacity of a conventional time-of-flight mass spectrometer.
In the ion source of an RF mass spectrometer, ions acquire energy eV and pass through a system of several stages arranged in series. Each stage consists of three spaced parallel grids. An RF voltage is applied to the intermediate grid. With the frequency of the applied RF field and energies eV being constant, only those ions can pass through the space between the first and intermediate grids that have a predetermined m/e ratio. The remaining ions are either retarded or acquire only insignificant energies and are repelled from the collected by means of a special decelerating electrode. Thus, only ions with the selected m/e ratio reach the collector. Therefore, in order to reset the mass spectrometer for registration of ions with a different mass, it is necessary either to change the initial energy of ion beam, or frequency of the RF field.
In a quadrupole mass spectrometer, ions are spatially redistributed in a transverse electric field with a hyperbolic distribution of the electric potential. This field is generated by a quadrupole capacitor having a d.c. voltage and RF voltage applied between pairs of rods. The ion beam is introduced into a vacuum chamber of the analyzer in the axial direction of the capacitor via an input opening. With the frequency and amplitude of the RF field being the same, only ions with a predetermined m/e ratio will have the amplitude of oscillations in the transverse direction of the analyzer shorter than the distances between the rods. Under the effect of its initial velocity, such ions will pass through the analyzer and will be registered and reach the collector, while all other ions will be neutralized on the rods and pumped out from the analyzer. Reset of such mass spectrometer to ions of another mass will require to change ether the amplitude or the frequency of the RF voltage. Quadrupole mass spectrometers have resolution capacity of about 103.
Magnetic resonance mass analyzers are based on the fact that the time required for ions to fly over a circular trajectory will depend on the ion mass. In such mass analyzers, resolution capacity reaches 2.5xc3x97104.
The last group relates to ion-cyclotron resonance mass spectrometers in which electromagnetic energy is consumed by ions, when cyclotron frequency of the ions coincides with the frequency of the alternating magnetic field in the analyzer. The ions move in a homogeneous magnetic field B along a spiral path with so-called cyclotron frequency
xcfx89c=eB/mc,
where c is velocity of light. At the end of their trajectory, the ions enter the collector. Only those ions reach the collector, the cyclotron frequency of which coincides with that of the alternating electric field in the analyzer. It is understood that selection of ions is carried out by changing the value of the magnetic field or of the frequency of the electromagnetic field. Ion-cyclotron resonance mass spectrometers ensure the highest resolution capacity. However, mass spectrometers of this type require the use of very high magnetic fields of high homogeneity, e.g., of 10 Tesla or higher. In other words, the system requires the use of super-conductive magnets which are expensive in cost and large in size.
Attempts have been made to improve existing mass spectrometers of the time-of-flight type, e.g., by improving ion storage devices, introducing deflectors for selection of ions for analysis in a mass spectrometer, reorganizing sequencing of ion packets or by extending the time of flight for improving resolution capacity of the mass spectrometers.
For example, U.S. Pat. No. 5,396,065 issued in 1995 to C. Myerholtz, et al. discloses an encoded sequence of ions in packets for use in time-of-flight mass spectrometers, in which the high-mass ions of a leading packet will be passed by the low-mass ions of a trailing packet. Thus, a high efficiency time-of-flight mass spectrometer is formed. The ions of each packet are acted upon to bunch the ions of the packet, thereby compensating for initial space and/or velocity distributions of ions in the launching of the packet. The times of arrival of the ions are determined at the detector to obtain a signal of overlapping spectra corresponding to the overlapping launched packets. A correlation between the overlapping spectra and the encoded launch sequence is employed to derive a single non-overlapped spectrum.
However, such method and apparatus makes interpretation of obtained data more complicated and not easily comprehensible. Furthermore, addition electronic circuits are required for control of the ion packet sequence.
U.S. Pat. No. 5,753,909 issued in 1998 to M. Park et al. describes a method and apparatus for analyzing ions by determining times of flight including using a collision cell to activate ions toward fragmentation and a deflector to direct ions away from their otherwise intended or parallel course. Deflectors are used as gates, so that particular ions may be selected for deflection, while others are allowed to continue along their parallel or otherwise straight path, from the ion source, through a flight tube, and eventually, to a detector. A postselector, in the form of two deflection plates is used as an ion deflector and is encountered by ions after the collision cell as they progress through the spectrometer.
A disadvantage of the device disclosed in U.S. Pat. No. 5,753,909 consists in that this mass spectrometers is based on the selection of specific ions and does not show the entire mass spectrum. For obtaining the entire spectrum, it is necessary to perform step by step scanning, and this requires an additional time.
U.S. Pat. No. 6,107,625 issued in 2000 to M. Park discloses a coaxial multiple reflection time-of-flight mass spectrometer of a time-of-flight type with resolution capacity improved due to a longer time of flight of the ions. The apparatus comprises two or more electrostatic reflectors positioned coaxially with respect to one another such that ions generated by an ion source can be reflected back and forth between them. The first reflecting device is an ion accelerator which functions as both an accelerating device to provide the initial acceleration to the ions, and a reflecting device to reflect the ions in the subsequent mass analysis. The second reflecting device is a reflectron which functions only to reflect the ions in the mass analysis. During the mass analysis, the ions are reflected back and forth between the accelerator and reflectron multiple times. Then, at the end of the ion analysis, either of the reflecting devices, preferably the ion accelerator, is rapidly deenergized to allow the ions to pass through that reflecting device and into a detector. By reflecting the ions back and forth between the accelerator and reflectron several times, a much longer flight path can be achieved in a given size spectrometer than could otherwise be achieved using the time-of-flight mass spectrometers disclosed in the prior art. Consequently, the mass resolving power of the time-of-flight mass spectrometer is substantially increased.
This is a typical system with storage of ions which does not allow a continuous mode of mass analysis since it requires period de-energization of one of the reflecting device. Obviously, the data is difficult to interpret, especially when masses of ions are scattered in a wide range so that light ions may undergo several reflection while heavy ions made only one or two reflections.
Thus, the existing mass spectrometers are either possess high performance characteristics at the expense of high cost and large dimensions, or are small in size, simple in construction, and inexpensive at the expense of the loss in resolution capacity and performance characteristics. None of the existing mass spectrometers combine in themselves such features as a reasonable cost, high performance characteristics, simple construction, and high resolution capacity.
It is an object of the present invention to provide a mass spectrometer that combines in itself such features as a reasonable cost, high performance characteristics, simple construction, and high resolution capacity. Another object is to provide a method of mass analyses, which allows to improve sensitivity and resolution capacity of a mass spectrometer. Another object is to provide a mass spectrometer operating in real time with convenient presentation of data for analysis. Still another object is to provide a mass spectrometer that combines advantages of dynamic time-of-flight systems with those of static mass spectrometers.
A mass spectrometer of the present invention is based on the use of quadrupole lenses with angular gradient of the electrostatic field. The device consists of an ion source connected to an ion mass separation chamber that contains a plurality of sequentially arranged coaxial electrostatic quadrupole lenses which generate a helical electrostatic field for sending ions along helical trajectories in a direct and return stroke. Scattering of positions of points of return is reduced by means of electrostatic mirrors located at the end of the direct stroke, while ions of different masses perform their return strokes along helical trajectories different from those of the direct strokes due to the use of a magnetic and/or electrostatic mirrors. An ion-electron emitting screen is installed on the path of ions in the reverse stroke, and positions of collision of the ions with the ion-electron emitting screen over time and space are detected with the use of micro-channel plate detectors. Movement of ions along the helical trajectory significantly increases the path of ions through the ion separation chamber and, hence, improves the resolution capacity of the mass spectrometer.